The Lithosphere–Asthenosphere Boundary in the North-West Atlantic Region
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Earth and Planetary Science Letters 236 (2005) 249–257 www.elsevier.com/locate/epsl The lithosphere–asthenosphere boundary in the North-West Atlantic region P. Kumar a,b, R. Kind a,c,*, W. Hanka a, K. Wylegalla a, Ch. Reigber a, X. Yuan a, I. Woelbern a, P. Schwintzer a,1, K. Fleming a, T. Dahl-Jensen d, T.B. Larsen d, J. Schweitzer e, K. Priestley f, O. Gudmundsson g, D. Wolf a aGeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany bNational Geophysical Research Institute, Uppal Road, Hyderabad-500007, India cFreie Universita¨t Berlin, Malteserstr. 74-100, 12249 Berlin, Germany dGeological Survey of Denmark, Oster Voldgade 10, 1350 Copenhagen K, Denmark eNORSAR, Institutsveien 25, 2027-Kjeller, Norway fUniv Cambridge, Bullard Lab, Madingley Rise, Cambridge, CB3 0EZ, United Kingdom gDanish Lithosphere Center, Oster Voldgade 10, 1350 Copenhagen K, Denmark Received 30 November 2004; received in revised form 21 April 2005; accepted 23 May 2005 Available online 1 July 2005 Editor: V. Courtillot Abstract A detailed knowledge of the thickness of the lithosphere in the north Atlantic is an important parameter for understanding plate tectonics in that region. We achieve this goal with as yet unprecedented detail using the seismic technique of S-receiver functions. Clear positive signals from the crust–mantle boundary and negative signals from a mantle discontinuity beneath Greenland, Iceland and Jan Mayen are observed. According to seismological practice, we call the negative phase the lithosphere–asthenosphere boundary (LAB). The seismic lithosphere under most of the Iceland and large parts of central Greenland is about 80 km thick. This depth in Iceland is in disagreement with estimates of the thickness of the elastic lithosphere (10–20 km) found from postglacial rebound data. In the region of flood basalts in eastern Greenland, which overlies the proposed Iceland plume track, the lithosphere is only 70 km thick, about 10 km less than in Iceland which is located directly above the proposed plume. At the western Greenland coast, the lithosphere * Corresponding author. GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany Tel.: +49 331 288 1240; fax: +49 331 288 1277. E-mail addresses: [email protected] (P. Kumar), [email protected] (R. Kind), [email protected] (W. Hanka), [email protected] (K. Wylegalla), [email protected] (X. Yuan), [email protected] (I. Woelbern), [email protected] (K. Fleming), [email protected] (T. Dahl-Jensen), [email protected] (T.B. Larsen), [email protected] (J. Schweitzer), [email protected] (K. Priestley), [email protected] (O. Gudmundsson), [email protected] (D. Wolf). 1 Has passed away on Dec. 2004. 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.05.029 250 P. Kumar et al. / Earth and Planetary Science Letters 236 (2005) 249–257 thickens to 100–120 km, with no indication of the Iceland plume track identified. Below Jan Mayen the lithospheric thickness varies between 40 and 60 km. D 2005 Elsevier B.V. All rights reserved. Keywords: Lithosphere–asthenosphere boundary; Iceland; Greenland; S-receiver functions 1. Introduction and adds a few more processing steps. Such steps are, as in the P-receiver function technique, source equal- High-viscosity lithospheric plates moving over a isation by deconvolution and distance move-out cor- lower-viscosity asthenosphere is a basic element of rection. Both steps are applied in order to enable the plate tectonics. Lithosphere and asthenosphere are summation of events from different distances and with originally mechanical definitions with regards to different magnitudes and source-time functions. This their reaction to forces acting over thousands or technique works very well, enabling observations of millions of years [1]. However, additional usages of the LAB with a resolution so far only known for the the term dlithosphereT have been introduced since Moho. then: thermal, seismic or chemical [2]. Obtaining In this work we determine the lithospheric thick- high-resolution seismic observations of the litho- ness for Iceland, Greenland and the island of Jan sphere–asthenosphere boundary (LAB) is not an Mayen. Greenland is a continent of Precambrian age easy task. Observations of low seismic velocities in (see [9] for a discussion of Greenland geology). the upper mantle are interpreted as being indicative of Darbyshire et al. [10] observed a thickening of the the asthenosphere. The first seismic observations of an lithosphere from 120 to 200 km going from east to dasthenospheric channelT were obtained by Gutenberg west in southern Greenland using the surface wave [3] at about 100 km depth. Therefore, seismologists technique. sometimes call the LAB the dGutenberg discontinuityT Iceland is thought to be one of the classic mantle (mostly in oceanic regions). The lithosphere is seis- plumes [11] interacting with a mid-ocean ridge, al- mologically divided into two parts, the crust and the though this view is disputed [12]. The crustal thick- mantle lithosphere, the latter being the high-velocity ness (up to 45 km) is several times thicker than that lid on top of the asthenosphere. However, high-reso- expected for oceanic crust (e.g. [13,14]). White and lution seismic body-wave observations of the LAB are McKenzie [15] concluded that the large thickness of very rare. This is in contrast to the Moho, which is the Icelandic crust is a result of magmatic intrusions. globally a much better documented discontinuity. So The mechanical lithosphere of Iceland is thought to be far, most information about the thickness of the lith- very thin (10–20 km), judging from rapid postglacial osphere comes from low-resolution surface-wave uplift [16]. A 10–20 km thick lithosphere would mean observations (e.g. [4]). The thickness of the litho- that the lower crust and Moho are located within the sphere is considered to be close to zero at mid- asthenosphere. There are also arguments that east ocean ridges, about 200 km beneath stable cratons, Iceland may be a continental splinter [17–19]. Seismic with 80–100 km being the global average. Thybo and surface-wave studies have furthermore found indica- Perchuc [5] suggest the existence of a global zone of tions of a 50–110 km thick lithosphere under parts of reduced velocity at about 100 km depth underlying Iceland [18,20,21]. Evans and Sacks [22] found be- continental regions, based on controlled source seis- tween Iceland and Jan Mayen a lithospheric thickness mic data. Li et al. [6] and Kumar et al. [7] obtained of 50 km from surface wave data, typical for young detailed maps of the LAB around the Hawaiian island oceans. Vinnik et al. [23] have shown a negative chain and in the Tien Shan–Karakoram region, respec- discontinuity at a depth of 80 km beneath all of Ice- tively, using the S-receiver function technique [8]. land, using essentially the same data and technique This new technique complements the traditional S to that we have used here. Jan Mayen is a small volcanic P conversion method (applied to S or SKS phases) island located about 600 km north of Iceland and P. Kumar et al. / Earth and Planetary Science Letters 236 (2005) 249–257 251 about 400 km east of Greenland on the Jan Mayen a fracture zone. It is considered to be a micro continent BRE [24]. HOT15 HOT09 66˚ KLU HOT12 HOT13 HOT29 HOT08 HOT07 HOT16 HOT06 ASBS HOT14 SKOT HOT04 HOT27 HOT17 HOT05 HOT18 BLOL HOT26 HOT25 2. Data and observations HOT03 BIRH HOT02 NYD ASKJ HOT24 HOT19 BORG HOT23 For the Greenland study, we used seismic data HOT30 HOFF LJOP KAF from the GLATIS and NEAT experiments [25,26]. 64˚ HOT21 During these experiments, seismic stations were HOT22 deployed along the Greenland coast and on the ice sheet for periods ranging from several months to -25˚ -20˚ -15˚ several years. For Iceland, we used the publicly avail- able data of the ICEMELT and HOTSPOT experi- ments [27,28]. In addition, we used data from 68˚ b permanent IRIS [29] and GEOFON [30] stations, 4 7 and from two seismic stations on the island of Jan 1 Mayen: JMI, operated by the Norwegian National Seismic Network (http://www.Ifjf.uib.no/Seismologi/ 66˚ nnsn/nsninfo2.html) and JMIC, operated by NOR- 2 5 8 SAR [31]. The locations of the stations and of the S-P piercing points at 80 km depth are shown in 64˚ Figs. 1–3. Since converted phases are usually weak 3 6 signals, a number of records must be summed to obtain a good signal-to-noise ratio. We defined non- overlapping regions on Greenland, Iceland and Jan 62˚ -25˚ -20˚ -15˚ -10˚ Fig. 2. a) Location of the temporary seismic stations of the HOT- SPOT and ICEMELT experiments and of the IRIS station BORG. b) Location of the piercing points of the S receiver functions at 80 km 80˚ I depth. Also marked are the regions (1–8) used for the summation of II ALE the seismic traces. E C DAG Mayen, where all traces with piercing points in 70˚ DBG UPN NGR these regions have been summed to form one record A F SUM HJO that is representative for the entire region (see Figs. GDH IS2 1–3). The number of traces stacked within each 60˚ SFJ SOE region are more than 20 (see Figs. 1, 2, and 3a). NUK DY2 D ANG Individual traces (not summed traces) are shown in B G 0˚ PAA Fig. 3b for the Jan Mayen stations as an example of the quality of our data. The individual S-receiver -60˚ -20˚ functions and its stacks are also shown from the -40˚ region 2 in Iceland (Fig.